1. Field of the Invention
The present invention relates to a method and apparatus for neutron dosimetry and more particularly to a method and apparatus for neutron dosimetry in a nuclear power plant in applications where on-line measurement of neutron flux is necessary or desirable.
2. Background of the Invention
The measurement of neutron exposure in selected regions within or outside of a nuclear reactor pressure vessel (RPV) is of interest for several reasons. Measurement of neutron exposure outside of the RPV can be used to obtain a determination of the plant operational power level. Measurement of neutron exposure outside of the RPV can also be used to determine the spatial power distribution within the RPV. Furthermore, measurement of neutron exposure within or outside of the RPV can provide dosimetry information with respect to fast neutron exposure, from which resulting vessel embrittlement may be inferred.
The foregoing constitutes a broad range of applications, for which a variety of devices and techniques are currently employed. Detectors positioned outside of the reactor core (typically BF.sub.3 counters) are used to detect thermal neutrons for plant power/power distribution measurements. Passive activation samples are used for radiometric inference of vessel fluence from within vessel measurements. Passive samples containing both radiometric activation samples and solid state track recorders (SSTR) are used in outside of vessel measurements for inference of vessel fluence.
Detectors positioned outside of the reactor pressure vessel are more accessible than those positioned within the RPV, but their removal can typically be effected only during a shutdown. While detectors so positioned are typically on-line devices, some are not. For example, passive counters must be transported to a laboratory in order to obtain data from them.
Thus, experience has taught that exceptionally useful features in neutron dosimetry include the following: (1) for power level/power distribution purposes, sufficient sensitivity to enable meaningful inference of integrated neutron fluence over a period of minutes to hours; (2) for vessel dosimetry applications, a lifetime of years in a high radiation (neutron or gamma ray radiation) environment, with a relative insensitivity to gamma radiation; (3) also for vessel dosimetry applications, a capability of periodic on-line readout of integrated neutron dose (fluence), to provide data useful in forming decisions concerning the operation of the plant (e.g., heat up/cool down rates); and (4) capability of providing independent measurements of thermal and fast neutron flux.